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CIE xyY Colour Space
It is a colour space derived from the CIEXYZ colour space as a cross between XYZ and its normalized coordinates x, y and z, such that the luminance Y is preserved. It is widely used to specify colours in practice.
In the conversion, the X and Z Tristimulus values are coverted to Chromaticity values x and y. The Y value is the lightness, which is independent of the hues.
This allows all colours to be represented as in 2-Dimensions using a chromaticity diagram, which is just the cross section of xyY space without considering the Y components.
Mathemtically, the colours of the chromaticity diagram occupies a region of the Real Projective Plane.